A nuclear reactor includes a pressure vessel having a substantially cylindrical side wall and a substantially hemispherical bottom wall above which a core vessel is suspended with its lower end spaced above this bottom wall. It is possible for the core vessel to accidentally fall, and since it is spaced for a substantial distance above the pressure vessel's bottom wall, it is possible for the latter to be ruptured and release the reactor core coolant.
To prevent this, the prior art has proposed the use of an intercept construction extending upwardly from the pressure vessel's bottom wall towards the core vessel's lower end for intercepting the core vessel if it accidentally falls. The construction is intended to limit, or substantially eliminate, the distance the core vessel can fall. Such a construction must transmit the falling force and ultimately the static weight of the core vessel to the pressure vessel's bottom wall.
Incidentally, in addition to preventing the core vessel from falling forcibly against the pressure vessel's wall in the event of an accident, it is important that the core vessel be intercepted so that it is held at least approximately at its normal position where it is within the coolant flow to assure that the core in the vessel continues to receive adequate cooling.
An example of a prior art core vessel intercept construction is described in the book VGB-Nuclear Power Plant Seminar 1970, p. 33. This construction is in effect a stool, and the force of holding the core vessel in position, in the event of an accident, is concentrated exclusively over a relatively small, localized area of the spherical bottom wall of the pressure vessel, giving rise to high local stressing of this bottom wall.